U.S. patent application number 11/445529 was filed with the patent office on 2007-12-06 for patterning of mechanical layer in mems to reduce stresses at supports.
Invention is credited to Wonsuk Chung, Ming-Hau Tung.
Application Number | 20070279753 11/445529 |
Document ID | / |
Family ID | 38789768 |
Filed Date | 2007-12-06 |
United States Patent
Application |
20070279753 |
Kind Code |
A1 |
Tung; Ming-Hau ; et
al. |
December 6, 2007 |
Patterning of mechanical layer in MEMS to reduce stresses at
supports
Abstract
A method of fabricating a MEMS device includes the formation of
support posts having horizontal wing portions at the edges of the
post. A mechanical layer is deposited over the support posts and
portions of the mechanical layer overlying portions of the support
post other than the horizontal wing portions are etched away. A
resultant MEMS device includes a mechanical layer overlying at
least a portion of the horizontal wing portions of the underlying
support structures.
Inventors: |
Tung; Ming-Hau; (San
Francisco, CA) ; Chung; Wonsuk; (San Jose,
CA) |
Correspondence
Address: |
KNOBBE, MARTENS, OLSON & BEAR, LLP
2040 MAIN STREET, FOURTEENTH FLOOR
IRVINE
CA
92614
US
|
Family ID: |
38789768 |
Appl. No.: |
11/445529 |
Filed: |
June 1, 2006 |
Current U.S.
Class: |
359/629 |
Current CPC
Class: |
B81C 2201/0109 20130101;
G02B 26/001 20130101; B81B 2201/047 20130101; B81C 1/00666
20130101 |
Class at
Publication: |
359/629 |
International
Class: |
G02B 27/14 20060101
G02B027/14 |
Claims
1-18. (canceled)
19. An MEMS device, comprising: an electrode layer located over a
substrate; at least one support structure; and a mechanical layer
located over said support structure and spaced apart from the
electrode layer by a cavity, the mechanical layer comprising an
aperture extending through said mechanical layer and overlying a
central portion of the support structure, the aperture being
surrounded by an annular section of the mechanical layer which
extends about the periphery of the support structure and overlies
the support structure.
20. The device of claim 19, wherein the support structure comprises
a substantially horizontal wing portion extending around the
periphery of the support structure.
21. The device of claim 20, wherein the mechanical layer overlies
only the substantially horizontal wing portion of the support
structures.
22. The MEMS device of claim 19, additionally comprising a
reflective layer located under the mechanical layer.
23. The MEMS device of claim 22, wherein the reflective layer
extends underneath a portion of the support structure.
24. The MEMS device of claim 22, wherein the reflective layer
extends over a portion of the support structure.
25. The MEMS device of claim 22, wherein the reflective layer
comprises Al.
26. The MEMS device of 20, additionally comprising a partially
reflective layer located over the substrate.
27. The MEMS device of claim 26, wherein the partially reflective
layer is located over the electrode layer.
28. The MEMS device of claim 19, additionally comprising a
dielectric layer located over the electrode layer.
29. The MEMS device of claim 20, wherein the support structure
comprises a sloped sidewall portion located in the interior of the
support structure, and a transition between said sloped sidewall
portion and the horizontal wing portion.
30. The MEMS device of claim 29, wherein the mechanical layer does
not overlie the transition between the sloped sidewall portion and
the horizontal wing portion.
31. The MEMS device of claim 29, wherein the mechanical layer
overlies the transition between the sloped sidewall portion and the
horizontal wing portion.
32. The MEMS device of claim 29, wherein the support structure
comprises a substantially flat base portion, and wherein the
aperture in the mechanical layer exposes the transition between the
sloped sidewall portion and the substantially flat base
portion.
33. The MEMS device of claim 19, additionally comprising: a
processor that is configured to communicate with at least one of
said electrode layer and said mechanical layer, said processor
being configured to process image data; and a memory device that is
configured to communicate with said processor.
34. The MEMS device of claim 33, further comprising a driver
circuit configured to send at least one signal to at least one of
said electrode layer and said mechanical layer.
35. The MEMS device of claim 34, further comprising a controller
configured to send at least a portion of the image data to the
driver circuit
36. The MEMS device of claim 33, further comprising an image source
module configured to send said image data to said processor.
37. The MEMS device of claim 36, wherein the image source module
comprises at least one of a receiver, transceiver, and
transmitter.
38. The MEMS device of claim 33, further comprising an input device
configured to receive input data and to communicate said input data
to said processor.
39. A MEMS device, comprising: first means for electrically
conducting; second means for electrically conducting; means for
supporting said second conducting means over said first conducting
means, wherein said second electrically conducting means overlies
only a peripheral portion of the supporting means, and wherein said
second conducting means is movable relative to said first
conducting means in response to generating electrostatic potential
between said first and second conducting means.
40. The MEMS device of claim 39, wherein: said first conducting
means comprise a lower electrode supported by a substrate; said
second conducting means comprise a movable upper electrode spaced
apart from the lower electrode; and said supporting means comprise
at least one support structure, wherein the upper electrode
comprises an aperture overlying at least a portion of the at least
one support structure, wherein an annular section of the upper
electrode that is defined by the aperture extends about the
periphery of the support structure and overlies the support
structure.
41. The MEMS device of claim 40, wherein the support structure
comprises a substantially horizontal wing portion, a sloped
sidewall portion, and a substantially flat base portion, and
wherein the aperture overlaps at least the transition between the
substantially flat base portion and the sloped sidewall
portion.
42. The MEMS device of claim 41, wherein the aperture further
overlaps at least the transition between the sloped sidewall
portion and the substantially horizontal wing portion.
43. An MEMS device, comprising: an electrode layer located over a
substrate; at least one support structure; a mechanical layer
located over said support structure and spaced apart from the
electrode layer by a cavity, the mechanical layer comprising an
aperture overlying at least a portion of the support structure,
wherein an annular section of the mechanical layer that is defined
by the aperture extends about the periphery of the support
structure and overlies the support structure; and a reflective
layer located under the mechanical layer, wherein the reflective
layer extends underneath a portion of the support structures.
44. An MEMS device, comprising: an electrode layer located over a
substrate; at least one support structure, wherein the support
structure comprises a substantially horizontal wing portion
extending wound the periphery of the support structure, a sloped
sidewall portion located in the interior of the support structure
and a transition between said sloped sidewall portion and the
horizontal wing portion, and a substantially flat base portion; and
a mechanical layer located over said support structure and spaced
apart from the electrode layer by a cavity, the mechanical layer
comprising an aperture overlying at least a portion of the support
structure, wherein an annular section of the mechanical layer that
is defined by the aperture extends about the periphery of the
support structure and overlies the support structure, and wherein
the aperture in the mechanical layer exposes the transition between
the sloped sidewall portion and the substantially flat base
portion.
Description
BACKGROUND OF THE INVENTION
[0001] Microelectromechanical systems (MEMS) include micro
mechanical elements, actuators, and electronics. Micromechanical
elements may be created using deposition, etching, and/or other
micromachining processes that etch away parts of substrates and/or
deposited material layers or that add layers to form electrical and
electromechanical devices. One type of MEMS device is called an
interferometric modulator. As used herein, the term interferometric
modulator or interferometric light modulator refers to a device
that selectively absorbs and/or reflects light using the principles
of optical interference. In certain embodiments, an interferometric
modulator may comprise a pair of conductive plates, one or both of
which may be transparent and/or reflective in whole or part and
capable of relative motion upon application of an appropriate
electrical signal. In a particular embodiment, one plate may
comprise a stationary layer deposited on a substrate and the other
plate may comprise a metallic membrane separated from the
stationary layer by an air gap. As described herein in more detail,
the position of one plate in relation to another can change the
optical interference of light incident on the interferometric
modulator. Such devices have a wide range of applications, and it
would be beneficial in the art to utilize and/or modify the
characteristics of these types of devices so that their features
can be exploited in improving existing products and creating new
products that have not yet been developed.
SUMMARY OF THE INVENTION
[0002] In one embodiment, a method of fabricating a
microelectromechanical systems (MEMS) device is provided, the
method including forming an electrode layer over a substrate,
depositing a sacrificial layer over the electrode layer, forming at
least one support structure, the support structure extending
through the sacrificial layers, where the support structure
includes a substantially horizontal wing portion extending around
the periphery of the support structure, depositing a mechanical
layer over the support structure, and patterning the mechanical
layer to form an aperture overlying a portion of the support
structure.
[0003] In another embodiment, a MEMS device is provided, including
an electrode layer located over a substrate, at least one support
structure, and a mechanical layer located over the support
structure and spaced apart from the electrode layer by a cavity,
the mechanical layer including an aperture overlying at least a
portion of the support structure, where an annular section of the
mechanical layer that is defined by the aperture extends about the
periphery of the support structure and overlies the support
structure.
[0004] In another embodiment, a MEMS device is provided, including
first means for electrically conducting, second means for
electrically conducting, and means for supporting said second
conducting means, wherein the second electrically conducting means
overlies only an exterior portion of the supporting means, and
wherein the second conducting means is movable relative to the
first conducting means in response to generating electrostatic
potential between the first and second conducting means.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] FIG. 1 is an isometric view depicting a portion of one
embodiment of an interferometric modulator display in which a
movable reflective layer of a first interferometric modulator is in
a relaxed position and a movable reflective layer of a second
interferometric modulator is in an actuated position.
[0006] FIG. 2 is a system block diagram illustrating one embodiment
of an electronic device incorporating a 3.times.3 interferometric
modulator display.
[0007] FIG. 3 is a diagram of movable mirror position versus
applied voltage for one exemplary embodiment of an interferometric
modulator of FIG. 1.
[0008] FIG. 4 is an illustration of a set of row and column
voltages that may be used to drive an interferometric modulator
display.
[0009] FIG. 5A illustrates one exemplary frame of display data in
the 3.times.3 interferometric modulator display of FIG. 2.
[0010] FIG. 5B illustrates one exemplary timing diagram for row and
column signals that may be used to write the frame of FIG. 5A.
[0011] FIGS. 6A and 6B are system block diagrams illustrating an
embodiment of a visual display device comprising a plurality of
interferometric modulators.
[0012] FIG. 7A is a cross section of the device of FIG. 1.
[0013] FIG. 7B is a cross section of an alternative embodiment of
an interferometric modulator.
[0014] FIG. 7C is a cross section of another alternative embodiment
of an interferometric modulator.
[0015] FIG. 7D is a cross section of yet another alternative
embodiment of an interferometric modulator.
[0016] FIG. 7E is a cross section of an additional alternative
embodiment of an interferometric modulator.
[0017] FIGS. 8A-8H are schematic cross-sections depicting certain
steps in the fabrication of an array of MEMS devices.
[0018] FIG. 8I is a top plan view of the array of MEMS devices
fabricated by the process of FIGS. 8A-8H.
[0019] FIGS. 9A-9C are schematic cross-sections depicting certain
alternate steps in the fabrication of an array of MEMS devices.
[0020] FIG. 9D is a top plan view of a portion of the array of MEMS
devices fabricated by the method of FIGS. 9A-9C.
[0021] FIG. 10A is a schematic cross-section of an alternate
embodiment of a MEMS device.
[0022] FIG. 10B is a top plan view of the MEMS device of FIG.
10A.
[0023] FIGS. 11A-11F are schematic cross-sections depicting certain
alternate steps in the fabrication of an array of MEMS devices.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0024] The following detailed description is directed to certain
specific embodiments of the invention. However, the invention can
be embodied in a multitude of different ways. In this description,
reference is made to the drawings wherein like parts are designated
with like numerals throughout. As will be apparent from the
following description, the embodiments may be implemented in any
device that is configured to display an image, whether in motion
(e.g., video) or stationary (e.g., still image), and whether
textual or pictorial. More particularly, it is contemplated that
the embodiments may be implemented in or associated with a variety
of electronic devices such as, but not limited to, mobile
telephones, wireless devices, personal data assistants (PDAs),
hand-held or portable computers, GPS receivers/navigators, cameras,
MP3 players, camcorders, game consoles, wrist watches, clocks,
calculators, television monitors, flat panel displays, computer
monitors, auto displays (e.g., odometer display, etc.), cockpit
controls and/or displays, display of camera views (e.g., display of
a rear view camera in a vehicle), electronic photographs,
electronic billboards or signs, projectors, architectural
structures, packaging, and aesthetic structures (e.g., display of
images on a piece of jewelry). MEMS devices of similar structure to
those described herein can also be used in non-display applications
such as in electronic switching devices.
[0025] In embodiments in which a MEMS device includes a mechanical
layer deposited over a support structure, such as a: support post,
mismatches in stresses within the mechanical layer and the support
post can result in undesirable flexure of the support post. In an
embodiment in which the MEMS device comprises an interferometric
modulator, a change in the unactuated position of the mechanical
layer will have an undesirable effect on the performance of the
MEMS device, such as a shift in the color reflected by the
interferometric modulator. In one embodiment, in which the support
structure has a non-flat upper surface, portions of the mechanical
layer not overlying the flat upper surface of the support post are
removed. This advantageously reduces the number of transitions in
the shape of the mechanical layer, which transitions would
exacerbate the undesired flexure of the edges of the support
posts.
[0026] One interferometric modulator display embodiment comprising
an interferometric MEMS display element is illustrated in FIG. 1.
In these devices, the pixels are in either a bright or dark state.
In the bright ("on" or "open") state, the display element reflects
a large portion of incident visible light to a user. When in the
dark ("off" or "closed") state, the display element reflects little
incident visible light to the user. Depending on the embodiment,
the light reflectance properties of the "on" and "off" states may
be reversed. MEMS pixels can be configured to reflect predominantly
at selected colors, allowing for a color display in addition to
black and white.
[0027] FIG. 1 is an isometric view depicting two adjacent pixels in
a series of pixels of a visual display, wherein each pixel
comprises a MEMS interferometric modulator. In some embodiments, an
interferometric modulator display comprises a row/column array of
these interferometric modulators. Each interferometric modulator
includes a pair of reflective layers positioned at a variable and
controllable distance from each other to form a resonant optical
cavity with at least one variable dimension. In one embodiment, one
of the reflective layers may be moved between two positions. In the
first position, referred to herein as the relaxed position, the
movable reflective layer is positioned at a relatively large
distance from a fixed partially reflective layer. In the second
position, referred to herein as the actuated position, the movable
reflective layer is positioned more closely adjacent to the
partially reflective layer. Incident light that reflects from the
two layers interferes constructively or destructively depending on
the position of the movable reflective layer, producing either an
overall reflective or non-reflective state for each pixel.
[0028] The depicted portion of the pixel array in FIG. 1 includes
two adjacent interferometric modulators 12a and 12b. In the
interferometric modulator 12a on the left, a movable reflective
layer 14a is illustrated in a relaxed position at a predetermined
distance from an optical stack 16a, which includes a partially
reflective layer. In the interferometric modulator 12b on the
right, the movable reflective layer 14b is illustrated in an
actuated position adjacent to the optical stack 16b.
[0029] The optical stacks 16a and 16b (collectively referred to as
optical stack 16), as referenced herein, typically comprise several
fused layers, which can include an electrode layer, such as indium
tin oxide (ITO), a partially reflective layer, such as chromium,
and a transparent dielectric. The optical stack 16 is thus
electrically conductive, partially transparent, and partially
reflective, and may be fabricated, for example, by depositing one
or more of the above layers onto a transparent substrate 20. The
partially reflective layer can be formed from a variety of
materials that are partially reflective such as various metals,
semiconductors, and dielectrics. The partially reflective layer can
be formed of one or more layers of materials, and each of the
layers can be formed of a single material or a combination of
materials.
[0030] In some embodiments, the layers of the optical stack 16 are
patterned into parallel strips, and may form row electrodes in a
display device as described further below. The movable reflective
layers 14a, 14b may be formed as a series of parallel strips of a
deposited metal layer or layers (orthogonal to the row electrodes
of 16a, 16b) deposited on top of posts 18 and an intervening
sacrificial material deposited between the posts 18. When the
sacrificial material is etched away, the movable reflective layers
14a, 14b are separated from the optical stacks 16a, 16b by a
defined gap 19. A highly conductive and reflective material such as
aluminum may be used for the reflective layers 14, and these strips
may form column electrodes in a display device.
[0031] With no applied voltage, the cavity 19 remains between the
movable reflective layer 14a and optical stack 16a, with the
movable reflective layer 14a in a mechanically relaxed state, as
illustrated by the pixel 12a in FIG. 1. However, when a potential
difference is applied to a selected row and column, the capacitor
formed at the intersection of the row and column electrodes at the
corresponding pixel becomes charged, and electrostatic forces pull
the electrodes together. If the voltage is high enough, the movable
reflective layer 14 is deformed and is forced against the optical
stack 16. A dielectric layer (not illustrated in this Figure)
within the optical stack 16 may prevent shorting and control the
separation distance between layers 14 and 16, as illustrated by
pixel 12b on the right in FIG. 1. The behavior is the same
regardless of the polarity of the applied potential difference. In
this way, row/column actuation that can control the reflective vs.
non-reflective pixel states is analogous in many ways to that used
in conventional LCD and other display technologies.
[0032] FIGS. 2 through 5B illustrate one exemplary process and
system for using an array of interferometric modulators in a
display application.
[0033] FIG. 2 is a system block diagram illustrating one embodiment
of an electronic device that may incorporate aspects of the
invention. In the exemplary embodiment, the electronic device
includes a processor 21 which may be any general purpose single- or
multi-chip microprocessor such as an ARM, Pentium.RTM., Pentium
II.RTM., Pentium III.RTM., Pentium IV.RTM., Pentium.RTM. Pro, an
8051, a MIPS.RTM., a Power PC.RTM., an ALPHA.RTM., or any special
purpose microprocessor such as a digital signal processor,
microcontroller, or a programmable gate array. As is conventional
in the art, the processor 21 may be configured to execute one or
more software modules. In addition to executing an operating
system, the processor may be configured to execute one or more
software applications, including a web browser, a telephone
application, an email program, or any other software
application.
[0034] In one embodiment, the processor 21 is also configured to
communicate with an array driver 22. In one embodiment, the array
driver 22 includes a row driver circuit 24 and a column driver
circuit 26 that provide signals to a display array or panel 30. The
cross section of the array illustrated in FIG. 1 is shown by the
lines 1-1 in FIG. 2. For MEMS interferometric modulators, the
row/column actuation protocol may take advantage of a hysteresis
property of these devices illustrated in FIG. 3. It may require,
for example, a 10 volt potential difference to cause a movable
layer to deform from the relaxed state to the actuated state.
However, when the voltage is reduced from that value, the movable
layer maintains its state as the voltage drops back below 10 volts.
In the exemplary embodiment of FIG. 3, the movable layer does not
relax completely until the voltage drops below 2 volts. Thus, there
exists a window of applied voltage, about 3 to 7 V in the example
illustrated in FIG. 3, within which the device is stable in either
the relaxed or actuated state. This is referred to herein as the
"hysteresis window" or "stability window." For a display array
having the hysteresis characteristics of FIG. 3, the row/column
actuation protocol can be designed such that during row strobing,
pixels in the strobed row that are to be actuated are exposed to a
voltage difference of about 10 volts, and pixels that are to be
relaxed are exposed to a voltage difference of close to zero volts.
After the strobe, the pixels are exposed to a steady state voltage
difference of about 5 volts such that they remain in whatever state
the row strobe put them in. After being written, each pixel sees a
potential difference within the "stability window" of 3-7 volts in
this example. This feature makes the pixel design illustrated in
FIG. 1 stable under the same applied voltage conditions in either
an actuated or relaxed pre-existing state. Since each pixel of the
interferometric modulator, whether in the actuated or relaxed
state, is essentially a capacitor formed by the fixed and moving
reflective layers, this stable state can be held at a voltage
within the hysteresis window with almost no power dissipation.
Essentially no current flows into the pixel if the applied
potential is fixed.
[0035] In typical applications, a display frame may be created by
asserting the set of column electrodes in accordance with the
desired set of actuated pixels in the first row. A row pulse is
then applied to the row 1 electrode, actuating the pixels
corresponding to the asserted column lines. The asserted set of
column electrodes is then changed to correspond to the desired set
of actuated pixels in the second row. A pulse is then applied to
the row 2 electrode, actuating the appropriate pixels in row 2 in
accordance with the asserted column electrodes. The row 1 pixels
are unaffected by the row 2 pulse, and remain in the state they
were set to during the row 1 pulse. This may be repeated for the
entire series of rows in a sequential fashion to produce the frame.
Generally, the frames are refreshed and/or updated with new display
data by continually repeating this process at some desired number
of frames per second. A wide variety of protocols for driving row
and column electrodes of pixel arrays to produce display frames are
also well known and may be used in conjunction with the present
invention.
[0036] FIGS. 4, 5A, and 5B illustrate one possible actuation
protocol for creating a display frame on the 3.times.3 array of
FIG. 2. FIG. 4 illustrates a possible set of column and row voltage
levels that may be used for pixels exhibiting the hysteresis curves
of FIG. 3. In the FIG. 4 embodiment, actuating a pixel involves
setting the appropriate column to -V.sub.bias, and the appropriate
row to +.DELTA.V, which may correspond to -5 volts and +5 volts,
respectively Relaxing the pixel is accomplished by setting the
appropriate column to +V.sub.bias, and the appropriate row to the
same +.DELTA.V, producing a zero volt potential difference across
the pixel. In those rows where the row voltage is held at zero
volts, the pixels are stable in whatever state they were originally
in, regardless of whether the column is at +V.sub.bias, or
-V.sub.bias. As is also illustrated in FIG. 4, it will be
appreciated that voltages of opposite polarity than those described
above can be used, e.g., actuating a pixel can involve setting the
appropriate column to +V.sub.bias, and the appropriate row to
-.DELTA.V. In this embodiment, releasing the pixel is accomplished
by setting the appropriate column to -V.sub.bias, and the
appropriate row to the same -.DELTA.V, producing a zero volt
potential difference across the pixel.
[0037] FIG. 5B is a timing diagram showing a series of row and
column signals applied to the 3.times.3 array of FIG. 2 which will
result in the display arrangement illustrated in FIG. 5A, where
actuated pixels are non-reflective. Prior to writing the frame
illustrated in FIG. 5A, the pixels can be in any state, and in this
example, all the rows are at 0 volts, and all the columns are at +5
volts. With these applied voltages, all pixels are stable in their
existing actuated or relaxed states.
[0038] In the FIG. 5A frame, pixels (1,1), (1,2), (2,2), (3,2) and
(3,3) are actuated. To accomplish this, during a "line time" for
row 1, columns 1 and 2 are set to -5 volts, and column 3 is set to
+5 volts. This does not change the state of any pixels, because all
the pixels remain in the 3-7 volt stability window. Row 1 is then
strobed with a pulse that goes from 0, up to 5 volts, and back to
zero. This actuates the (1,1) and (1,2) pixels and relaxes the
(1,3) pixel. No other pixels in the array are affected. To set row
2 as desired, column 2 is set to -5 volts, and columns 1 and 3 are
set to +5 volts. The same strobe applied to row 2 will then actuate
pixel (2,2) and relax pixels (2,1) and (2,3). Again, no other
pixels of the array are affected. Row 3 is similarly set by setting
columns 2 and 3 to -5 volts, and column 1 to +5 volts. The row 3
strobe sets the row 3 pixels as shown in FIG. 5A. After writing the
frame, the row potentials are zero, and the column potentials can
remain at either +5 or -5 volts, and the display is then stable in
the arrangement of FIG. 5A. It will be appreciated that the same
procedure can be employed for arrays of dozens or hundreds of rows
and columns. It will also be appreciated that the timing, sequence,
and levels of voltages used to perform row and column actuation can
be varied widely within the general principles outlined above, and
the above example is exemplary only, and any actuation voltage
method can be used with the systems and methods described
herein.
[0039] FIGS. 6A and 6B are system block diagrams illustrating an
embodiment of a display device 40. The display device 40 can be,
for example, a cellular or mobile telephone. However, the same
components of display device 40 or slight variations thereof are
also illustrative of various types of display devices such as
televisions and portable media players.
[0040] The display device 40 includes a housing 41, a display 30,
an antenna 43, a speaker 45, an input device 48, and a microphone
46. The housing 41 is generally formed from any of a variety of
manufacturing processes as are well known to those of skill in the
art, including injection molding and vacuum forming. In addition,
the housing 41 may be made from any of a variety of materials,
including, but not limited to, plastic, metal, glass, rubber, and
ceramic, or a combination thereof. In one embodiment, the housing
41 includes removable portions (not shown) that may be interchanged
with other removable portions of different color, or containing
different logos, pictures, or symbols.
[0041] The display 30 of the exemplary display device 40 may be any
of a variety of displays, including a bi-stable display, as
described herein. In other embodiments, the display 30 includes a
flat-panel display, such as plasma, EL, OLED, STN LCD, or TFT LCD
as described above, or a non-flat-panel display, such as a CRT or
other tube device, as is well known to those of skill in the art.
However, for purposes of describing the present embodiment, the
display 30 includes an interferometric modulator display, as
described herein.
[0042] The components of one embodiment of the exemplary display
device 40 are schematically illustrated in FIG. 6B. The illustrated
exemplary display device 40 includes a housing 41 and can include
additional components at least partially enclosed therein. For
example, in one embodiment, the exemplary display device 40
includes a network interface 27 that includes an antenna 43, which
is coupled to a transceiver 47. The transceiver 47 is connected to
a processor 21, which is connected to conditioning hardware 52. The
conditioning hardware 52 may be configured to condition a signal
(e.g., filter a signal). The conditioning hardware 52 is connected
to a speaker 45 and a microphone 46. The processor 21 is also
connected to an input device 48 and a driver controller 29. The
driver controller 29 is coupled to a frame buffer 28 and to an
array driver 22, which in turn is coupled to a display array 30. A
power supply 50 provides power to all components as required by the
particular exemplary display device 40 design.
[0043] The network interface 27 includes the antenna 43 and the
transceiver 47 so that the exemplary display device 40 can
communicate with one or more devices over a network. In one
embodiment, the network interface 27 may also have some processing
capabilities to relieve requirements of the processor 21. The
antenna 43 is any antenna known to those of skill in the art for
transmitting and receiving signals. In one embodiment, the antenna
transmits and receives RF signals according to the IEEE 802.11
standard, including IEEE 802.11(a), (b), or (g). In another
embodiment, the antenna transmits and receives RF signals according
to the BLUETOOTH standard. In the case of a cellular telephone, the
antenna is designed to receive CDMA, GSM, AMPS, or other known
signals that are used to communicate within a wireless cell phone
network. The transceiver 47 pre-processes the signals received from
the antenna 43 so that they may be received by and further
manipulated by the processor 21. The transceiver 47 also processes
signals received from the processor 21 so that they may be
transmitted from the exemplary display device 40 via the antenna
43.
[0044] In an alternative embodiment, the transceiver 47 can be
replaced by a receiver. In yet another alternative embodiment, the
network interface 27 can be replaced by an image source, which can
store or generate image data to be sent to the processor 21. For
example, the image source can be memory device such as a digital
video disc (DVD) or a hard-disc drive that contains image data, or
a software module that generates image data.
[0045] The processor 21 generally controls the overall operation of
the exemplary display device 40. The processor 21 receives data,
such as compressed image data from the network interface 27 or an
image source, and processes the data into raw image data or into a
format that is readily processed into raw image data. The processor
21 then sends the processed data to the driver controller 29 or to
the frame buffer 28 for storage. Raw data typically refers to the
information that identifies the image characteristics at each
location within an image. For example, such image characteristics
can include color, saturation, and gray-scale level.
[0046] In one embodiment, the processor 21 includes a
microcontroller, CPU, or logic unit to control operation of the
exemplary display device 40. The conditioning hardware 52 generally
includes amplifiers and filters for transmitting signals to the
speaker 45, and for receiving signals from the microphone 46. The
conditioning hardware 52 may be discrete components within the
exemplary display device 40, or may be incorporated within the
processor 21 or other components.
[0047] The driver controller 29 takes the raw image data generated
by the processor 21 either directly from the processor 21 or from
the frame buffer 28 and reformats the raw image data appropriately
for high speed transmission to the array driver 22. Specifically,
the driver controller 29 reformats the raw image data into a data
flow having a raster-like format, such that it has a time order
suitable for scanning across the display array 30. Then the driver
controller 29 sends the formatted information to the array driver
22. Although a driver controller 29, such as a LCD controller, is
often associated with the system processor 21 as a stand-alone
Integrated Circuit (IC), such controllers may be implemented in
many ways. They may be embedded in the processor 21 as hardware,
embedded in the processor 21 as software, or fully integrated in
hardware with the array driver 22.
[0048] Typically, the array driver 22 receives the formatted
information from the driver controller 29 and reformats the video
data into a parallel set of waveforms that are applied many times
per second to the hundreds and sometimes thousands of leads coming
from the display's x-y matrix of pixels.
[0049] In one embodiment, the driver controller 29, the array
driver 22, and the display array 30 are appropriate for any of the
types of displays described herein. For example, in one embodiment,
the driver controller 29 is a conventional display controller or a
bi-stable display controller (e.g., an interferometric modulator
controller). In another embodiment, the array driver 22 is a
conventional driver or a bi-stable display driver (e.g., an
interferometric modulator display). In one embodiment, a driver
controller 29 is integrated with the array driver 22. Such an
embodiment is common in highly integrated systems such as cellular
phones, watches, and other small area displays. In yet another
embodiment, the display array 30 is a typical display array or a
bi-stable display array (e.g., a display including an array of
interferometric modulators).
[0050] The input device 48 allows a user to control the operation
of the exemplary display device 40. In one embodiment, the input
device 48 includes a keypad, such as a QWERTY keyboard or a
telephone keypad, a button, a switch, a touch-sensitive screen, or
a pressure- or heat-sensitive membrane. In one embodiment, the
microphone 46 is an input device for the exemplary display device
40. When the microphone 46 is used to input data to the device,
voice commands may be provided by a user for controlling operations
of the exemplary display device 40.
[0051] The power supply 50 can include a variety of energy storage
devices as are well known in the art. For example, in one
embodiment, the power supply 50 is a rechargeable battery, such as
a nickel-cadmium battery or a lithium ion battery. In another
embodiment, the power supply 50 is a renewable energy source, a
capacitor, or a solar cell including a plastic solar cell, and
solar-cell paint. In another embodiment, the power supply 50 is
configured to receive power from a wall outlet.
[0052] In some embodiments, control programmability resides, as
described above, in a driver controller which can be located in
several places in the electronic display system. In some
embodiments, control programmability resides in the array driver
22. Those of skill in the art will recognize that the
above-described optimizations may be implemented in any number of
hardware and/or software components and in various
configurations.
[0053] The details of the structure of interferometric modulators
that operate in accordance with the principles set forth above may
vary widely. For example, FIGS. 7A-7E illustrate five different
embodiments of the movable reflective layer 14 and its supporting
structures. FIG. 7A is a cross section of the embodiment of FIG. 1,
where a strip of metal material 14 is deposited on orthogonally
extending supports 18. In FIG. 7B, the moveable reflective layer 14
is attached to supports 18 at the corners only, on tethers 32. In
FIG. 7C, the moveable reflective layer 14 is suspended from a
deformable layer 34, which may comprise a flexible metal. The
deformable layer 34 connects, directly or indirectly, to the
substrate 20 around the perimeter of the deformable layer 34. These
connections are herein referred to as support posts or structures
18. The embodiment illustrated in FIG. 7D has support post
structures 18 that include support plugs 42 upon which the
deformable layer 34 rests. The movable reflective layer 14 remains
suspended over the cavity, as in FIGS. 7A-7C, but the deformable
layer 34 does not form the support posts by filling holes between
the deformable layer 34 and the optical stack 16. Rather, the
support posts 18 are formed of a planarization material, which is
used to form the support post plugs 42. The embodiment illustrated
in FIG. 7E is based on the embodiment shown in FIG. 7D, but may
also be adapted to work with any of the embodiments illustrated in
FIGS. 7A-7C, as well as additional embodiments not shown. In the
embodiment shown in FIG. 7E, an extra layer of metal or other
conductive material has been used to form a bus structure 44. This
allows signal routing along the back of the interferometric
modulators, eliminating a number of electrodes that may otherwise
have had to be formed on the substrate 20.
[0054] In embodiments such as those shown in FIG. 7, the
interferometric modulators function as direct-view devices, in
which images are viewed from the front side of the transparent
substrate 20, the side opposite to that upon which the modulator is
arranged. In these embodiments, the reflective layer 14 optically
shields the portions of the interferometric modulator on the side
of the reflective layer opposite the substrate 20, including the
deformable layer 34. This allows the shielded areas to be
configured and operated upon without negatively affecting the image
quality. Such shielding allows the bus structure 44 in FIG. 7E,
which provides the ability to separate the optical properties of
the modulator from the electromechanical properties of the
modulator, such as addressing and the movements that result from
that addressing. This separable modulator architecture allows the
structural design and materials used for the electromechanical
aspects and the optical aspects of the modulator to be selected and
to function independently of each other. Moreover, the embodiments
shown in FIGS. 7C-7E have additional benefits deriving from the
decoupling of the optical properties of the reflective layer 14
from its mechanical properties, which are carried out by the
deformable layer 34. This allows the structural design and
materials used for the reflective layer 14 to be optimized with
respect to the optical properties, and the structural design and
materials used for the deformable layer 34 to be optimized with
respect to desired mechanical properties.
[0055] In one embodiment, a method of manufacturing an array of
MEMS devices, such as the interferometric modulators described
above, is described with respect to FIGS. 8A-8H. In FIG. 8A, it can
be seen that an electrode layer 52 has been deposited on a
substrate 50, and that a partially transparent or partially
reflective layer 54 has been deposited over the electrode layer 52.
The partially reflective layer 54 and the electrode layer 52 are
then patterned and etched to form gaps 56 which may define strip
electrodes formed from the electrode layer 52. In addition, the gap
56 may define the location where a support structure will be
formed. In one embodiment, the electrode layer 52 comprises
indium-tin-oxide (ITO). In one embodiment, the partially reflective
layer 54 comprises a layer of chromium (Cr). In other embodiments,
the placement of the layers 52 and 54 may be reversed, such that
the partially reflective layer is located underneath the electrode
layer 54. In another embodiment, a single layer (not shown) may
serve as both the electrode layer and the partially reflective
layer. In other embodiments, only one of the electrode layer 52 or
the partially reflective layer 54 may be formed.
[0056] In FIG. 8B, it can be seen that a dielectric layer 58 has
been deposited over the patterned electrode layer 52 and partially
reflective layer 54. In one embodiment, the dielectric layer 58 may
comprise SiO.sub.2. In further embodiments, one or more etch stop
layers (not shown) may be deposited over the dielectric layer.
These etch stop layers may protect the dielectric layer during the
patterning of overlying layers. In one embodiment, a etch stop
layer comprising Al.sub.2O.sub.3 may be deposited over the
dielectric layer 58. In a further embodiment, an additional layer
of SiO.sub.2 may be deposited over the etch stop layer.
[0057] In FIG. 8C, a sacrificial layer 60 has been deposited over
the dielectric layer 58. In one embodiment, the sacrificial layer
60 comprises molybdenum (Mo) or silicon (Si), but other materials,
such as tungsten (W), may be appropriate. Advantageously, the
sacrificial layer 60 is selectively etchable with respect to the
layers surrounding the sacrificial layer 60. As can also be seen in
FIG. 8C, a movable layer, in the illustrated embodiment taking the
form of a reflective layer 62, has been deposited over the
sacrificial layer. In certain embodiments, this movable layer will
comprise a conductive material. In the illustrated embodiment,
unlike the partially reflective layer 54, the reflective layer 62
need not transmit any light through the layer, and thus
advantageously comprises a material with high reflectivity. In one
embodiment, the reflective layer 62 comprises aluminum (Al) or
aluminum alloys, as aluminum has both very high reflectivity and
acceptable mechanical properties. In other embodiments, specular
materials such as silver and gold may be used in the reflective
layer 62. In further embodiments, particularly in non-optical MEMS
devices in which the movable layer need not be reflective, other
materials, such as nickel and copper may be used in the movable
layer.
[0058] In FIG. 8D, the sacrificial layer 60 and the reflective
layer 62 have been patterned and etched to form apertures 64 which
extend through the sacrificial and reflective layers 60 and 62. As
can be seen in the illustrated embodiment, these apertures 64 are
preferably tapered to facilitate continuous and conformal
deposition of overlying layers.
[0059] With respect to FIG. 8E, it can be seen that a post layer 70
has been deposited over the patterned reflective layer 62 and
sacrificial layer 60. This post layer 70 will form support posts
located throughout an array of MEMS devices. In embodiments in
which the MEMS devices being fabricated comprise interferometric
modulator elements (such as modulator elements 12a and 12b of FIG.
1), some of the support posts (such as the support structures 18 of
FIG. 1) will be located at the edges of the upper movable
electrodes (such as the movable reflective layer 14 of FIG. 1) of
those interferometric modulator elements. In addition, support
posts may also be formed in the interior of the resulting
interferometric modulator elements, away from the edges of the
upper movable electrode, such that they support a central or
interior section of the upper movable electrode. In one embodiment,
the post layer 70 comprises SiO.sub.2, but a wide variety of post
materials, such as SiN.sub.x, may also be used. In certain
embodiments, the post layer 70 may comprise an inorganic material.
Preferably, the post layer 70 is conformally and continuously
deposited, particularly over the apertures 64 (FIG. 8D).
[0060] In FIG. 8F, it can be seen that the post layer 70 has been
patterned and etched to form a post structure 72. In addition, it
can be seen that the illustrated post structure 72 has a peripheral
portion which extends horizontally over the underlying layers; this
horizontally-extending peripheral portion will be referred to
herein as a wing portion 74. As with the patterning and etching of
the sacrificial layer 60, the edges 75 of the post structure 72 are
preferably tapered or beveled in order to facilitate deposition of
overlying layers with better step coverage and reduced stress. In
the illustrated embodiment, the post structure 72 further comprises
a sloped sidewall portion 88 and a substantially flat base portion
90. It can be seen that a transition 76a exists between the
substantially horizontal wing portion 74 and the sloped sidewall
portion 88, and that a transition 76b exists between the sloped
sidewall portion 88 and the substantially flat base portion 90.
[0061] Because the reflective layer 62 was deposited prior to the
deposition of the post layer 70, it will be seen that the
reflective layer 62 may serve as an etch stop during the etching
process used to form the post structure 72, as the portion of the
post structure 72 being etched is isolated from the underlying
sacrificial layer 60 by the reflective layer 62, even though other
portions of the post layer 70 are in contact with the sacrificial
layer 60. Thus, an etch process can be used to form the post
structures 72 which would otherwise also etch the sacrificial layer
60.
[0062] Variations to the above process may be made, as well. In one
embodiment, the reflective layer may be deposited after the
patterning and etching of the sacrificial layer, such that the post
layer may be completely isolated from the sacrificial layer, even
along the sloped sidewalls of the apertures in the sacrificial
layer. Such an embodiment provides an etch stop protecting the post
structure during the release etch to remove the sacrificial layer.
In another embodiment, the post layer may be deposited over a
patterned sacrificial layer prior to the deposition of the
reflective layer, as will be understood from the description of
FIGS. 11A-11F below. Such an embodiment may be used if the
sacrificial layer will not be excessively consumed during the
etching of the post structure, even without an etch stop.
[0063] In FIG. 8G, it can be seen that a mechanical layer 78 has
been deposited over the post structures 72 and the exposed portions
of the reflective layer 62. As the reflective layer 62 provides the
reflective portion of the interferometric modulator element, the
mechanical layer 78 may advantageously be selected for its
mechanical properties, such as the modulus of elasticity, without
regard for the reflectivity. In one embodiment, the mechanical
layer 78 advantageously comprises nickel (Ni), although various
other materials, such as Al, may be suitable. For convenience, the
combination of the mechanical layer 78 and reflective layer 62 may
be referred to collectively as the deformable electrode or
deformable reflective layer 80.
[0064] After deposition of the mechanical layer 78, the mechanical
layer 78 is patterned and etched to form desired structures. In
particular, the mechanical layer 78 may be patterned and etched to
form gaps which define electrodes formed from strips of the
mechanical layer which are electrically isolated from one another.
The underlying reflective layer 62 may also be patterned and etched
to remove the exposed portions of the reflective layer 62. In one
embodiment, this may be done via a single patterning and etching
process. In other embodiments, two different etches may be
performed in succession, although the same mask used to pattern and
etch the mechanical layer 78 may be left in place and used to
selectively etch the reflective layer 62. In one particular
embodiment, in which the mechanical layer 68 comprises Ni and the
reflective layer 62 comprises Al, the Ni may be etched by a nickel
etch (which generally comprise nitric acid, along with other
components), and the Al may be etched by either a phosphoric/acetic
acid etch or a PAN (phosphoric/acetic/nitric acid) etch. A PAN etch
may be used to etch Al in this embodiment, even though it may etch
the underlying sacrificial layer 62 as well, since the deformable
reflective layer 80 has already been formed over the sacrificial
layer 62, and the desired inter-electrode spacing has thus been
obtained. Any extra etching of the sacrificial layer 62 during this
etch will not have a detrimental effect on the finished
interferometric modulator.
[0065] In FIG. 8H, it can be seen that the deformable electrode or
reflective layer 80, which comprises the mechanical layer 78 and
the reflective layer 62, has also been patterned and etched to form
etch holes 82. A release etch is then performed to selectively
remove the sacrificial layer 60 (FIG. 8G), forming a cavity 84
which permits the deformable reflective layer 80 to deform toward
the electrode layer 52 upon application of appropriate voltage. In
one embodiment, the release etch comprises a fluorine-based etch,
such as a XeF.sub.2 etch, which will selectively remove sacrificial
materials like Mo, W, or silicon without significantly attacking
surrounding materials such as Al, SiO.sub.2, Ni, or
Al.sub.2O.sub.3. The etch holes 82, along with gaps 81 between the
strip electrodes 83 (see FIG. 81) formed from the mechanical layer
78, advantageously permit exposure of the sacrificial layer 60 to
the release etch.
[0066] FIG. 8I depicts an overhead view of an array of MEMS
devices, which include support structures 72 such as the support
structure 72 of FIG. 8H. As can be seen, two upper strip electrodes
83, which are separated by a gap 81, completely overlay certain
support structures 72 located underneath the upper electrodes 83.
In the illustrated embodiment, certain other support structures 73
extend across the gap 81, supporting two upper electrodes 83. As
can be seen in FIG. 8I, the shape of these edge support structures
73, may differ from the shape of the interior support structures 72
which are located underneath the upper electrode 83. As can also be
seen, etch holes 82 extend through the upper electrodes 83. In
other embodiments (not shown), support structures may not extend
across the gap 81, but may rather be positioned near the edges of
the electrodes 83, such that a support structure provides support
for a single electrode 83 near the edge of the electrode next to
the gap 81.
[0067] As can be seen in FIG. 8H, the height of the sacrificial
layer 60 determines the height of the cavity 84 resulting from the
removal of the sacrificial layer 60. As discussed above, in an
embodiment in which MEMS devices being fabricated comprise
interferometric modulators, the height of the cavity determines the
color reflected by the interferometric modulator in the unactuated,
or relaxed, state illustrated in FIG. 8H. Thus, it is important
that the deformable electrode or reflective layer 80 remains at a
predetermined position (e.g., at a particular distance from the
partially reflective layer 54) when the interferometric modulator
element is in a relaxed state. Preferably, during deposition of the
mechanical layer 78 as described with respect to FIG. 8G, the
mechanical layer 78 is formed such that it contains residual
tensile stress. This residual tensile stress will tend to pull the
mechanical layer 78, and therefore the deformable electrode or
reflective layer 80, into the illustrated substantially flat
position between support posts.
[0068] However, in multilayer structures, these residual stresses
can also be problematic. In particular, undesired flexure of the
support structures and the mechanical layer overlying the support
structures may occur as a result of unbalanced stresses within
support structures such as post structures 72 and the mechanical
layer 78. In some situations, these unbalanced stresses are the
result of inherent stresses within the materials forming the
support structures 72 and the mechanical layer 78. An additional
source of unbalanced stresses is the thermal expansion of the
layers in response to changes in temperature, which is a function
of the mismatch between the coefficients of thermal expansion of
two different materials, the operating temperatures of the MEMS
device, the moduli of elasticity of the materials, and the material
deposition conditions during fabrication of the MEMS device. When
adjoining layers have different coefficients of thermal expansion,
deflection may not only be caused by the relative change in size of
adjoining layers, but the total deflection may vary as a result of
the operating temperature. Because such deflection will alter the
height of the interferometric cavity, and therefore the color
reflected by the interferometric modulator element, it is desirable
to eliminate or minimize such deflection to the amount
possible.
[0069] This deflection is exacerbated by the shapes of the support
structure 72 and the overlying mechanical layer 78. In the
illustrated embodiment, as can be seen in FIGS. 8H and 8I, the
support structure 72 comprises the horizontal wing portion 74, as
well as a sloped sidewall portion 88 and a substantially flat base
portion 90. The mechanical layer 78 which overlies these portions
of the support structure 72 will be contoured to the same shape, so
long as the mechanical layer 78 comprises a non-planarizing
material (e.g., CVD or PVD formed). There are thus two transitions
in the mechanical layer 78 as it moves from the substantially flat
base portion 90 of the support structure 72, up the sloped sidewall
portion 88, and over the wing portion 74. These transitions, at
which both the mechanical layer 78 and support post 72 bend, will
result in additional undesired flexure of portions of the
interferometric modulator due to stress mismatches and changes in
size of the mechanical layer 78. These transitions correspond to
the transitions 76a and 76b in the support structure 72 discussed
with respect to FIG. 8F, and occur because the mechanical layer 78
in the embodiment of FIG. 8H extends over both transitions 76a,76b
in the support structure 72.
[0070] In one embodiment, the risk of deflection of the support
structures due to mismatches between the support structures and the
mechanical layer overlying those support structures may be
minimized through the selective removal of certain portions of the
mechanical layer. In one embodiment, discussed with respect to
FIGS. 9A-9C, a method for fabricating such a MEMS device includes
the steps discussed with respect to FIGS. 8A-8G.
[0071] In FIG. 9A, it can be seen that the mechanical layer 78 has
been patterned and etched to remove portions of the mechanical
layer 78 located over the support structure 72, and has also
simultaneously been etched to form etch holes 82 extending through
the mechanical layer 78. As discussed above, the mechanical layer
78 may be further etched at this point to form other structures,
including the etching of gaps 81 in the mechanical layer 78 which
will define upper strip electrodes 83 (see FIG. 9D). In the
illustrated embodiment, the portions of the mechanical layer 78
overlying both the sloped sidewall portion 88 and the substantially
flat base portion 90 have been removed to form an aperture 91
through the mechanical layer, so that the mechanical layer 78 is in
contact with and supported by only the wing portions 74 of the
support structure 72. A portion 77 of the flat upper wing portion
74 is exposed by this aperture 91. Such an embodiment
advantageously avoids the overlaying of the mechanical layer 78
over both the transition 76a between the substantially horizontal
wing portion 74 and the sloped sidewall portion 88 and the
transition 76b between the sloped sidewall portion 88 and the
substantially flat base portion 90, minimizing the deflection that
may occur due to the mismatch.
[0072] Now, in FIG. 9B, it can be seen that the portions of the
reflective layer 62 exposed by the etch holes 82 extending through
the mechanical layer 78 (see FIG. 9A) are removed, such that the
etch holes 82 now extend through both the mechanical layer 78 and
the reflective layer 62, exposing portions of the underlying
sacrificial layer 60. It will be understood that in certain
embodiments, depending on the composition and thickness of the
deposited and underlying layers, the mechanical layer 78 and the
reflective layer 62 may be patterned and etched in a single
process.
[0073] In FIG. 9C, the sacrificial layer 60 (see FIG. 9B) has been
removed, such as by a release etch process. As discussed above,
this release etch may comprise a fluoride-based etch, such as a
xenon difluoride etch. A released MEMS device is thus formed,
having a support post 72 supporting a mechanical layer 78, wherein
the mechanical layer 78 extends over only a portion of the support
post 72. In the illustrated embodiment, the mechanical layer 78
extends only over the substantially horizontal wing portion of the
support post 72. It will be understood that while the illustrated
support post 72 is an interior support post, and is surrounded by
the mechanical layer 72 on all sides, other support posts, referred
to as edge posts, may extend into the gaps between upper strip
electrodes.
[0074] FIG. 9D depicts an overhead view of an array of MEMS
devices, which include support structures 72 such as the support
structure 72 of FIG. 9C. It can be seen that the array of FIG. 9D
differs from that of FIG. 81 in that the upper electrode 83, which
comprises the mechanical layer 78 (see FIG. 9C), overlies only the
substantially horizontal wing portions 74 of the interior support
structure 72 and the edge support structure 73. The sloped sidewall
portion 88 and the base 90 of the support structure are exposed due
to the formation of aperture 91 in the mechanical layer 78 and
reflective layer 62(see FIG. 9B). In the illustrated embodiment, a
narrow annular inner portion 77 of the wing portions 74 is exposed
by the aperture 91, such that the transition or corner 76a (see
FIG. 9C) between the wing portions 74 and the sidewall portion 88
is exposed.
[0075] In the embodiment of FIGS. 9C and 9D, the mechanical layer
78 extends over only parts of the wing portion 74 of the support
structure 72. However, in other embodiments, the mechanical layer
78 may still extend over other portions of the support structure
72. One particular embodiment is described with respect to FIGS.
10A and 10B.
[0076] FIG. 10A shows a cross-section of a fabricated MEMS device,
in which it can be seen that the mechanical layer 78 has been
patterned to form an aperture 91 overlying a portion of the support
structure 72, such that the mechanical layer extends down a portion
of the sloped sidewall portion 88, such that the mechanical layer
78 overlays only the upper transition 76a in the shape of the
support structure 72. The extent to which a mechanical layer 78 can
extend over the support post 72 without resulting in an undesired
amount of deflection will depend on a variety of factors, including
but not limited to the thickness and rigidity of the various
layers, particularly the support post, and the operating
temperature of the MEMS device. In the illustrated embodiment, it
can be seen that the mechanical layer 78 does not overlay the lower
transition 76b between the sloped sidewall portion 88 and the
substantially flat base portion 90.
[0077] FIG. 10B depicts an overhead view of the MEMS device of FIG.
10A. As can be seen, the mechanical layer 78 overlies the wing
portion 74 and part of the sloped sidewall portion 88, while the
aperture 91 in the mechanical layer 78 overlies the base 90 of the
support structure 72. Thus, the upper transition 76a is covered by
the mechanical layer 78 while the lower transition 76b is exposed
by the aperture 91 in the mechanical layer 78.
[0078] As discussed above, in another embodiment, the reflective
layer may be deposited at a different point in the fabrication
process. In one embodiment, a method for fabricating a MEMS device
includes the steps described with respect to FIGS. 8A-8B. In FIG.
11A, it can be seen that a sacrificial layer 60 has been deposited
over the dielectric layer, and patterned and etched to form
apertures 64, which are preferably tapered as illustrated in the
figure, as discussed above.
[0079] In FIG. 11B, it can be seen that a layer of post material 70
is deposited over the sacrificial layer 60. In certain embodiments,
particularly those in which the post layer 70 is selectively
etchable relative to the sacrificial layer 60, the post layer 70
may be deposited directly over the sacrificial layer 60. In other
embodiments, however, an etch stop layer (not shown) may be
deposited between the sacrificial layer 60 and the post layer 70,
permitting the post layer 70 to be etched without substantially
consuming the sacrificial layer 60. In certain embodiments, the
etch stop layer may be left in the finished MEMS device, but in
other embodiments, in the etch stop layer may be removed, usually
during or after the release etch.
[0080] In FIG. 11C, the post layer 70 is patterned and etched as
discussed above to create post structures 72. In the illustrated
embodiment, the support structures 72 comprise a substantially flat
base portion 90, a sloped sidewall portion 88, and a substantially
horizontal wing portion 74 extending around the periphery of the
support structure 72. It can also be seen that in the illustrated
embodiments, the outer edge 75 of the wing portion 74 is
tapered.
[0081] In FIG. 11D, a reflective layer 62 and mechanical layer 78
have been deposited over the support structure 72. In contrast to
the device of FIG. 8H, it can be seen that the partially fabricated
device of FIG. 11D will not include a reflective surface on the
underside of the wing portion 74 of the support structure 72. In
certain embodiments, this may be desirable, as the portion of the
reflective layer 62 located underneath the wing portion 74 in FIG.
8H will generally remain at a substantially fixed distance from the
partially reflective layer 54, even when the deformable reflective
layer or electrode 80 is in an actuated position. This may result
in undesirable optical interference.
[0082] In FIG. 11E, the mechanical layer 78 is patterned and etched
as discussed above with respect to FIG. 9B, removing those portions
of the mechanical layer 78 that overlie the sloped sidewall portion
88 and base 90 of the support structure 72, as well as forming etch
holes 82 extending through the mechanical layer 78. Both the upper
transition 76a and the lower transition 76b are thus exposed. The
reflective layer 72 is also patterned and etched, removing those
portions of the reflective layer 72 exposed by the removal of the
portions of the mechanical layer 78, and exposing portions of the
underlying sacrificial layer 60.
[0083] Finally, in FIG. 11F, the sacrificial layer 60 (see FIG.
11E) is removed. A released MEMS device is thus formed, in which a
support post 72 has a wing portion 74 extending around the edges of
the support post 72 and supporting the overlying reflective layer
62 and mechanical layer 78. As can be seen in both this embodiment
and the embodiment of FIG. 9C, the MEMS devices further comprise a
portion of the reflective layer 62 located either over or
underneath the wing portion 74 of the support structure 72. While
this portion of the reflective layer 62 may contribute to the
stress mismatch, in both embodiments, the thickness of the
reflective layer 62 will generally be thin relative to the
thicknesses of the support post 72 and mechanical layer 78, such
that even in embodiments (not pictured) in which the reflective
layer 62 extends either over or under the sloped sidewall region 88
or base 90, the amount of deflection caused by this stress mismatch
will be negligible. Thus, because the reflective layer may be thin
relative to the surrounding layers, in certain embodiments, the
reflective layer may remain over the upper surface of the support
structure, while portions of the mechanical layer overlying the
reflective layer and the post are removed. In certain other
embodiments, either a portion of the reflective layer or a portion
of both the reflective layer and the mechanical layer may extend at
least partially down the sloped sidewall portion 88 of the support
post 74, similar to the embodiments discussed with respect to FIGS.
10A and 10B.
[0084] Various modifications may be made to the above process
flows. In particular, depending on the composition of the various
layers and the etches used, the order in which certain layers are
deposited can be varied. It will also be recognized that the order
of layers and the materials forming those layers in the above
embodiments are merely exemplary. In addition, although in the
illustrated embodiment, the support structures are generally
depicted as round or having rounded corners, in alternate
embodiments the support structures may have different shapes.
Moreover, in some embodiments, other layers, not shown, may be
deposited and processed to form portions of a MEMS device or to
form other structures on the substrate. In other embodiments, these
layers may be formed using alternative deposition, patterning, and
etching materials and processes, may be deposited in a different
order, or composed of different materials, as would be known to one
of skill in the art.
[0085] It is also to be recognized that, depending on the
embodiment, the acts or events of any methods described herein can
be performed in other sequences, may be added, merged, or left out
altogether (e.g., not all acts or events are necessary for the
practice of the methods), unless the text specifically and clearly
states otherwise.
[0086] For example, in other embodiments, the processes and
structures described above with respect to FIGS. 9A-10F may be used
in conjunction with the embodiments of FIGS. 1-7E, and in
particular the various interferometric modulator structures
described with respect to those figures. In other embodiments, the
processes and structures described above may be used in conjunction
with MEMS devices which need not have the optical properties
described above, such as MEMS switches.
[0087] While the above detailed description has shown, described,
and pointed out novel features of the invention as applied to
various embodiments, it will be understood that various omissions,
substitutions, and changes in the form and details of the device of
process illustrated may be made by those skilled in the art without
departing from the spirit of the invention. As will be recognized,
the present invention may be embodied within a form that does not
provide all of the features and benefits set forth herein, as some
features may be used or practiced separately from others.
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